Jan Jansen PhD, Abstract Co-Author: Nothing to Disclose

Paul C. Shrimpton PhD, Presenter: Nothing to Disclose

Study influence of scanner model and its operation on organ and effective doses for standard CT examinations of the brain, chest, abdomen and pelvis when normalised to CTDI in free air (CTDIFIA) and standard CT dosimetry phantoms (CTDIW).

Estimates of typical organ and effective dose are often required to support justification and optimisation of CT examinations. Monte Carlo simulation represents a powerful tool for such dosimetry and we have performed series of calculations to model absorbed doses from 11 scanners covering 4 manufacturers to 3 anthropomorphic reference adult patients: HPA18+, an updated mathematical phantom reflecting the 2007 ICRP Recommendations; and the ICRP voxel phantoms Adult Male and Adult Female. Irradiated slice thicknesses of 1 cm were modelled for series of contiguous simulations along the length of each phantom. Further calculations modelled the absorbed dose to air, both on axis in free air and within standard head and body CT dosimetry phantoms for the calculation of CTDIW.

Analyses for each standard examination and reference patient included 68 combinations of CT scanner model and operating conditions. Choice of reference patient affected normalised values of effective dose by up to 40%. Normalising effective dose to CTDIW rather than CTDIFIA led to reduced coefficients of variation in the data for a given examination/ phantom combination: ranges of 8-12% versus 17-22%, respectively.

Normalised organ and effective doses for CT examinations depend critically on the reference patient utilised for their assessment and adoption of a standardised approach would facilitate comparisons of typical doses. Detailed CT dosimetry also requires due account of scanner model and conditions of operation, although normalisation of such organ doses to CT monitoring quantities (CTDIW, CTDIVol or DLP) can extend their application to allow broad estimates of typical doses for a wider range of CT scanning.

This work will support CT centres in their estimates of organ and effective doses for typical CT examinations based on measurements of standard CT dose quantities.

Jansen, J, Shrimpton, P, Influence of the CT Scanner on Normalised Organ and Effective Doses for Standard Examinations Estimated by Monte Carlo Simulation for Three Adult Reference Patients.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11004752.html